Process for labelling sulfur-containing compounds

A process for obtaining the replacement by isotopes of at least one of the hydrogen atoms selected from the group consisting of those attached to the .alpha.- or .alpha.'-carbon atom in a heterocyclic compound containing the structural moiety (I) ##STR7## as an integral part of a heterocyclic ring selected from the group consisting of 1,3-oxathiolane, 1,3-oxathiane, 1,3-dithiolane and 1,3-dithiane, in preference to isotopic replacement of hydrogen atoms attached to carbon atoms in said compound other than said .alpha.- and .alpha.'-carbon atoms, which process comprises the steps of oxidizing the compound to the corresponding sulfoxide or sulfone which contains the corresponding structural moiety (II) ##STR8## wherein in the sulfoxide n is 1 and in the sulfone n is 2, effecting the isotopic replacement by a known method of said at least one hydrogen atom, and reducing the resultant isotope-labelled sulfoxide or sulfone to the corresponding isotopically labelled compound containing the structural moiety (I), in which labelled compound isotopes of hydrogen are attached to at least one carbon atom selected from the group consisting of said .alpha.- and .alpha.'-carbon atoms.